「すみません、おなまえ　わすれてしまったんですが、もういちど　きいてもいいですか？」

「I'm sorry, May I ask your name again cause I forgot.」

がいいとおもいます。

is good I think.

そして、がいこくじんだとつかえるいいわけとしては、

I think there is a useful excuse for foreigner for this situation is

「にほんじん の なまえ を おぼえるの は むずかしくて、、、」といいながら、すこし もうしわけないかお を したらいいとおもいます！

「It's hard to remember Japanese name...」and express that you feel sorry/ashame for that.

すみません おなまえなんでしたっけ？

なまえおぼえるの にがてなんですよ *smile*

oh sorry what was your name?

l’m terrible at remembering names. *smile*

A lot of Japanese natives say stuff like that. (Including me) It’s more friendly and less offensive!

I hope this will help!

What i actually do is that i protend to ask for their fathers name but they think that i’m asking dor their name. For example i’d be like” oh hey, what’s your name?” And when they answer it, i’d be like nah, i meant ur fathers name. 

Or sometime i tell them stright away and i apologize for it. We’re human after all 

O-namae-wa nan deshita ka?

お名前はなんでしたか？

Is what I would use since it is easy to remember and polite. I don't know if it would be appropriate to begin with a "sumimasen" before asking for their name again, or if there is a better way to ask if you want to be more informal.

Hey, thanks Michael, Chiaki-san, Aoi-san and Abraham.

Lots of good advice and great ideas. Much obliged.

Gary